Improving Source Positions in the OCARS Catalog: A First Approach

Abstract Optical Characteristics of Astrometric Radio Sources (OCARS) is a compiled catalog of various additional data associated with astrometric radio sources whose coordinates have been determined from very long baseline interferometry observations. It contains source coordinates, object type, redshift, optical, and near-infrared magnitudes. Until now, OCARS source coordinates were simply copied from input catalogs and, as a result, were systematically inhomogeneous. This work is the first attempt to obtain a unified set of radio source coordinates aligned to the International Celestial Reference Frame (ICRF), more especially to the third ICRF release, ICRF3. Comparison of the source coordinates in the old OCARS version as of 2024 December and the new OCARS version as of 2025 March with the ICRF3-SX catalog using the vector spherical harmonics technique showed almost complete elimination of systematic errors in new OCARS positions relative to the ICRF3 frame.